Electrostatic resin powder spray system

ABSTRACT

An electrostatic spray resin coating system recirculates overspray resin particles which do not contact the workpiece and blends them with new resin powder in a supply hopper. A predetermined small volume of powder is maintained in the hopper which contains a distribution of resin powder particle sizes that will not suffer elutriative separation, assures uniform coating, and prevents obstructions caused by compaction of the powder. Means for detecting the workpiece opposite the spray nozzle controls the supply of powder to the nozzle and minimizes the amount of overspray which contains a high proportion of coarse particles, and powder level detecting means responsive to the volume of new and recirculated powder in the supply hopper controls the supply of new powder mixed with the overspray in the supply hopper.

United States Patent [191 Diamond et a1.

ELECTROSTATIC RESIN POWDER SPRAY SYSTEM Inventors: Harold G. Diamond,Wexford; Floyd L. Gustine, Pittsburgh, both of Pa.

Allis-Chalmers Manufacturing Company, Milwaukee, Wis.

Filed: May 28, 1970 Appl. No.: 41,228

Related U.S. Application Data Continuationdnpart of Ser. No. 722,078,April 17, 1968, abandoned, and a continuation-in-part of Ser. No.728,253, May 10, 1968, abandoned.

Assignee:

U.S. Cl 118/7, 118/312, 118/603, 118/634 Int. Cl B05b 5/02 Field ofSearch... 118/2, 7, 312, 326, 603, 631, 118/308, 633, 634, 635, 630;51/8, 12

References Cited UNITED STATES PATENTS 2,770,212 11/1956 Marantz 118/312X 3,480,330 11/1969 Hirs et a1. 118/312 X 2,759,449 8/1956 LindquisL118/312 X 3,014,451 12/1961 Rhodes.... 118/312 X 3,439,650 4/1969Stowell 118/603 X Primary Examiner.lohn P. McIntosh Attorney, Agent, orFirm-Lee l-l. Kaiser [57] ABSTRACT An electrostatic spray resin coatingsystem recirculates overspray resin particles which do not contact theworkpiece and blends them with new resin powder in a supply hopper. Apredetermined small volume of powder is maintained in the hopper whichcontains a distribution of resin powder particle sizes that will notsuffer elutriative separation, assures uniform coating, and preventsobstructions caused by compaction of the powder. Means for detecting theworkpiece opposite the spray nozzle controls the supply of powder to thenozzle and minimizes the amount of overspray which contains a highproportion of coarse particles, and powder level detecting meansresponsive to the volume of new and recirculated powder in the supplyhopper controls the supply of new powder mixed with the overspray in thesupply hopper.

VALVE MOTOR CONTROL PATENTEDFEBI 2 3.791341 SHEET 1 UF 4 l AIR EXHAUS'I"SPRAY BOOTH 2| PATENIEB FEB I 2 I874 SHEEI 0F 4 MOTOR CONTROLELECTROSTATIC RESIN POWDER SPRAY SYSTEM This application is acontinuation-in-part of our parem application r l N! 72, ZZ8 .-nti ld 1.5199 2- static Resin Powder Spray System, filed April 17, 1968, no wabandon ed and also of our continuationin-part application Serial No.728,253, filed May ltl,

1968 ha the s m e, n9waba td9 ed-.W

This invention relates to apparatus for coating metallic articles with aresinous material.

The metallic covers of electrical apparatus such as distributiontransformers are often covered with a relatively thick resin coating toprevent animals and birds for coming in contact with the metal and beingelectrocuted. The resin is conventionally a combination of specificmaterials such as epoxy resins and polyamide resins and is usuallyapplied by a fluidized bed process in which the metallic article to becoated 'is heated to a temperature above the melting point of the resinand immersed into a fluidized powdered resin mass to cause the resinparticles to melt and adhere to the heated metallic article and to flowinto a smooth uniform resin coating on the article.

Resin powder particles may vary in size between 0.2 and 500 microns. The0.2 micron sizes are easily airborne, and at constant air velocity theyare quickly removed from the fluidized bed, leaving increasing amountsof coarse fractions behind. When the major amount left is coarse andcannot be airborne without greatly increasing the air velocity,volcanoes and blowholes occur which result in uneven coating andstreaking of the resin coating on the metal article. The curing cyclefor flowing out the powder into a uniform coating is sensitive to thepresence of a sufficient volume of fine resin particles to initiatemelting at a low and constant temperature, and the imbalance of particlesizes may change the curing cycle and necessitate scrapping of largeamounts of resin powder.

This fluid flow separation of particle sizes is known as elutriation. Itis a disadvantage in fluidized bed operations. Elutriation is avoided inthe present invention by means for keeping the air flow and velocityconstant for a given empirically determined small volume of mixed sizeparticles so that all of the sizes are airborne and expelled through thespray gun before elutriative separation can occur. The overspray, whichcontains a higher proportion of coarse particles than virgin powder, isalso kept to small increments for blending back into the smallincrements of virgin powder in the system so as to maintain a totalvolume for electrostatic spraying that contains a size distribution thatwill not suffer elutriative separation at the air volume and velocityemployed. The invention thus preserves the balance of the powder sizingwhich is necessary for uniform and optimum resincoating.

Electrostatic spray resin coating systems are known which spray theresin powder from nozzles onto the heated article as it is movedrelative to the nozzles. In known electrostatic spray resin coatingsystems the resin particles which do not contact the heated metalarticle, termed the overspray, are collected and recirculated, thedebris and dirt are removed from the recirculated powder ina separator,the recovered powder is introduced into a container such as a fluidizedbed where it is mixed with new powder, and the resulting mixture is thenfed into hoses leading to the spray nozzles. In addition to the fineparticles settling in such fluidized bed, it was found that therecovered overspray contains agglomerated resin particles which tend toaccumulate near the bottom of the bed. Such accumulation of agglomeratedparticles often grows until the entire contents of the bed, which maycontain several hundred pounds of powder, must be scrapped.

Conventional electrostatic spray resin coating systems have numerousshutdowns and require the continued presence of an operator. Suchshutdowns are usually caused by obstructions resulting from compactionof the powder which block circulation of the resin particles, and theexpense of the continuous presense of an operator and the downtime ofthe spray coating apparatus substantially increases the cost of resincoating. Further the percentage of resin powder particles which areactually used in coating the metal article is relatively low inconventional systems, and the air surrounding such conventional spraysystems is usually contaminated with the resin powder particles.

It is impractical in such an electrostatic resin spray coating system toshut off the powder, and the powder spray is controlled by shutting offthe pressurized air. The resin powder is usually fed by gravity to theinjection point in a venturi chamber and is carried away during the timethat the air flows through the central jet in the venturi. When thepressurized air is shut off, the resin powder continues to flow bygravity and fills the venturi chamber and passages connected thereto.The extremely fine resin particles appear to continue to flow when theair is shut off and to compact on the walls of the venturi to the extentthat they block the passageway for the powder after only a few days ofoperation. The extremely fine resin particles find their way into thescrew threads behind the O-ring gasketed ram, which controls the amountof powder mixed with the air, so that they jam the threads and preventadjustment of the ram. The resin particles accumulate and adhere to thewalls of the venturi to such an extent that it is impossible to forcesufficient powder through the venturi for spray coating even when theair pressure is raised from the normal operating range of 15-20 poundsper square inch to pounds per square inch, and the, powder adheres sotenaciously to the venturi walls that a cutting tool is required toremove the compacted particles.

We have found that an electrostatic spray resin coating system whichutilizes a large volume of resin powder has many more shutdownsresulting from compacted powder obstructions than one which uses a smallvolume of powder; that the overspray contains a much higher percentageof coarse resin particles, i.e., of approximately mesh size, than thenew powder; that such overspray resin particles tend to clog theconventional powder separator and the main supply hopper which feeds thespray nozzles if a large volume is permitted to accumulate even when theoverspray powder is mixed with new powder; that a high percentage ofsuch coarse overspray particles in the powder fed to the spray nozzleschanges the curing cycle of the resin and results in streaking andinferior coatings; that such coarse overspray particles settle andaccumulate near the bottom of a fluidized bed used to feed the spraynozzles and result in feeding powder to the nozzles which does not havea uniform distribution of particle sizes and may necessitate scrappingof the entire bed;

that the resin coating produced in a system having a fluidized bed feedfor the spray nozzles often streaks and contains voids which make itunsatisfactory for applications where the coating must have highdielectric strength; and that an optimum resin coating without voids isobtained when a uniform distribution of powder particle sizes is fed tothe nozzles which results in a coating having the closest possiblepacking of powder particles.

It is an object of the invention to provide an improved electrostaticspray resin coating system which uses a minimum amount of resin powder.

A further object of the invention is to provide such a resin powdercoating system which utilizes a minimum amount of resin powder and keepsthe powder continuously in motion and prevents agglomeration oraccumulation of sufficient volume of powder at any point in thecirculating system to cause compaction of the powder and consequentobstruction of the system.

A still further object is to provide such an improved electrostaticspray resin coating system which utilizes a sufficiently small volume ofresin powder so that all particle sizes are expelled through the spraygun before elutriative separation can occur and a balanced mixture ofresin powder particle sizes is sprayed on the article to be coated.

Another object is to provide such an electrostatic spray resin powdercoating system which maintains a constant proportion of cleanedrecirculated powder to new powder and feeds a balanced mixture of resinparticle sizes to the spray nozzles at all times.

Still another object of the invention is to provide such anelectrostatic spray resin coating system which supplies a uniform blendof powder particle sizes to the spray nozzles at all times and producesan optimum and uniform coating without voids and streaking and havinghigh dielectric strength.

Another object of the invention is to provide such an improvedelectrostatic spray resin coating system which substantially eliminatesshutdowns caused by obstructions resulting from compacted powderparticles and which permits continuous operation without the presence ofan operator.

It is a still further object of the invention to provide such animproved electrostatic spray resin coating system wherein only a veryminute portion of the resin powder particles is not actually used incoating the article and wherein the air surrounding the sprayingapparatus is relatively clean in comparison to known systems. 1

These and other objects and advantages of the invention will be morereadily apparent from the following detailed description when consideredin conjunction with the accompanying drawing wherein:

FIG. I is an elevation view, partially schematic, of an electrostaticspray resin powder coating system embodying our invention;

FIG. 2 is a side elevation view of the powder dispensing means of thesystem of FIG. 1;

FIGS. 3 and 4 are front and side elevation views respectively of thedump valve of the apparatus of FIGS. 1 and 2 with portions broken awayto better illustrate the internal construction; i

FIG. 5 is a front elevation view of the improved venturi means of theapparatus of FIG. 1;

FIG. 6 is a section view through the venturi shown in FIG. 5;

FIG. 7 is an elevation view, partly schematic, of an alternativeembodiment of electrostatic spray resin.

coating system incorporating our invention;

FIG. 8 is a schematic plan view of the powder separator of the system ofFIG. 7;

FIG. 9 is an elevation view, partly schematic, of another alternativeembodiment of our invention; and

FIG. 10 is an enlarged partial view taken on line X-X of FIG. 9;

Referring to FIG. 1, the metallic article, or workpiece to be coatedwith resin is shown as a distribution transformer cover 10 which hasbeen heated above the melting temperature of the resin to be applied andis being moved through a spray booth 11 on an overhead conveyorincluding an I-beam 12, movable support means 13 having wheels 14engaging the lower flange of the I-beam 12, and hook means 15 dependingfrom the movable support means and engaging the cover 10. The workpiece10 is schematically shown to be grounded, and the resin particlesaccumulate an elec trostatic charge as they pass through the spraynozzles 17, which are connected to a suitable high potential sourceshown schematically as a battery 19, and are attracted to the groundedworkpiece 10. The resin particles which do not contact the workpiece 10are withdrawn through ducts 21 from the upper and lower ends of spraybooth I 1 vertically upward into a powder sepa rator 23 by suitable aircirculating means shown schematically as a fan 24 disposed at theentrance to an air exhaust duct 25 on the housing of separator 23. Therecovered powder particles are returned by gravity from separator 23through a duct 28 to a recirculated powder screening bin 30 where theypass through a grate type hopper magnet 32 which attracts anyferromagnetic particles therein. The recovered resin particles fall ontoan inclined vibrating screen 33 actuated by an electromagnet 35, and anyagglomerated resin particles or debris which do not pass through thescreen 33 slide down the vibrating screen and fall into a sight glasscontainer 37.

The recirculated resin powder particles which pass through screen 33fall into a generally cone-shaped powder supply hopper 39. The wallsdefining hopper 39 are relatively flexible and are inclined at a steepslope and may have steel plates 40 welded thereto which are cyclicallyattracted by electromagnetic vibrators 42 to flex and vibrate the hopperwalls and thus cause the powder to fall into the bottom of the hopper.

The throat at the lower end of a new powder hopper 44 positionedadjacent the recirculated powder bin 30 registers with the aperture 45(see FIG. 4) in the upper end of a motor-operated dump valve 47. Dumpvalve 47 is cylindrical and has a plurality of elongated powderreceiving depressions 49 in its outer periphery which extend parallel toits axis. Dump valve 47 is rotated by an electric motor 52 through asuitable gear speed reducer 54 and belt and pulley means 55. The newresin powder is introduced into new hopper 44 and falls into theelongated powder receiving depressions 49 in dump valve 47, and theamount of new powder delivered through aperture 56 at the lower end ofdump valve 47 into powder supply hopper 39 is determined by the rate ofrotation of dump valve 47.

A bin level control 57 on a sidewall of powder supply hopper 39 providesan electrical output signal to a dump motor valve control means 59,shown schematically in block form, when the amount of new andrecirculated resin powder in supply hopper 39 is below a levelindicating that a predetermined amount of powder, preferablyapproximately five pounds, is in supply hopper 39. In response to theoutput signal from bin level control 57, valve control means 59energizes motor 52 to rotate dump valve 47 and introduce new powder intosupply hopper 39 until this predetermined amount is reached, at whichtime bin level control 57 removes the electrical signal to valve motorcontrol means 59 to deenergize motor 52.

The mixture of new and recirculated resin powder in supply hopper 39falls by gravity into the venturi chamber 60 within the housing 61 ofeach of a plurality of venturis 62. A suitable supply of pressurized airis connected by conduits 63 to the venturis 62, and the pressurized airflows through an air inlet passage 64 in each housing 61 and thencethrough a radial aperture 65 in an adjustable ram 66 disposed within acompartment in housing 61. Ram 66 has a smaller diameter portion 67 withO-ring gaskets at each end thereof which permits the air to flow intoradial aperture 65 regardless of the ram position and thence through anaxial aperture 68 in ram 66 into the venturi chamber 60 to force theresin powder out of chamber 60 through a diverging outlet passage 69 inhousing 61 and into flexible hoses 70 connected to the venturis and tothe spray nozzles 17. Ram 66 has external threads 71 which engageinternal threads in housing 61 to permit axial movement of the ram 66into and out of venturi chamber 60, when it is rotated by knurled knob72, to control the amount of resin powder which mixes with the air.

The venturi housing 61 and ram 66 are preferably of an insulatingplastic material such as polytetrafluorethylene having nonstickingproperties. It has been found that venturis 62 constructed of suchmaterial substantially prevent adhesion and compacting of the fine resinpowder and do not clog even after months of operation.

We have found that an electrostatic spray resin coating system whichutilizes a large volume of resin powder has frequent shutdowns resultingfrom obstructions caused by compacted powder, and FIG. 7 illustrates analternative embodiment of our invention which permits a smaller amountof powder to be in the system than the apparatus of FIGS. 1-6, theelements similar to those of the system of FIGS. 1-6 being given thesame reference numerals. The source of pressurized air is shown as apump 75 connected to conduit 63 and an adjustable regulating valve 77which may be adjusted to vary the pressure of air supplied to venturis62 and thus control the volume of resin powder from the nozzles 17.Means schematically shown in FIG. 7 for minimizing the amount of resinpowder in the electrostatic spray resin coating system includes anormally closed valve 79 in conduit 63 operable to open position by anelectrical operating coil 80 to supply pressurized air to venturis 62when the coil 80 is energized and means responsive to the presence of anarticle to be coated opposite the spray nozzles 17 for completing anenergizing circuit to operating coil 80. The means for completing anenergizing circuit to operating coil 80 is schematically shown as anormally open limit switch 81 in series with a source of electricalpower 82 and the coil 80 and an operating arm 84 for switch 81 in thepath of movement of the conveyor support means 13 adapted whenengaged bysupport means 13 to operate limit switch 81 to closed position and thuscomplete the energizing circuit to coil to thereby open valve 79 andsupply pressurized air to venturis 62. It will be appreciated that suchmeans will shut off the supply of resin powder when no workpiece 10 isopposite the spray nozzles 17 and thus minimize the amount of powderrecirculated in the system.

The powder separator 23 preferably has a plurality of dependinginflatable bags 27 within a housing 86. Each bag 27 has a fluid venturiopening 87 at the open upper end thereof, and means are provided tointermittently puff pressurized air into each bag 27 to inflate it andthus cause overspray resin particles that have settled on the bag tofall by gravity into the lower end of housing 86. The means forintermittently inflating bags 27 schematically shown in FIGS. 7 and 8includes a source of pressurized air such as a pump 90, a manifold 92receiving pressurized air from pump 90, a plurality of headers 93 eachof which communicates with manifold 92 and is disposed above the openend of a plurality of bags 27 and has a plurality of orifices (notshown) therein each of which registers with the venturi opening 87 atthe upper end of one of the bags 27, a normally closed valve 95 betweeneach header 93 and manifold 92, a pilot valve 96 associated with eachvalve 95 having an operating coil (not shown) and being adapted toactuate the corresponding valve 95 to open position when its operatingcoil is energized, a rotary electrical switch having a plurality ofstationary contacts 98 arranged in a circle with each stationary contact98 corinected by conductor means to the operating coil of one of thepilot valves 96, a movable contact 100 adapted when rotated tosequentially engage the stationary contacts 98 and being connected toone side of a source of electrical power 102, and means such as a motor103 for rotating movable contact 100. It will be appreciated that whenmovable contact 100 rotates, the pilot valves 96 are sequentiallyoperated for sufficient time to open the valves 95 and cause pressurizedair to flow through the headers 93 and the orifices therein into theventuris opening 87 to intermittently inflate the bags 27.

A rotatable cylindrical dump valve 104 at the lower end of powderseparator 23 is similar to dump valve 47. described hereinbefore and iscontinuously rotated by a motor 106 to prevent accumulation ofsufficient overspray powder in separator 23 to result in compaction ofthe powder which might result in obstruction of the recirculationsystem. Cylindrical dump valve 104 is disposed between opposed,elongated, arcuate-in-cross section depressions 107 in the valve housingwalls and has a close fit therewith which permits fan 24 to develop arelatively large negative pressure in housing 86 and ducts 21 to drawthe overspray resin particles from booth 11 into powder separatorhousing 86. At its lower end powder separator housing 86 has arelatively flexible, inverted conical sidewall 108 of steep slope toprevent accumulation of the overspray resin particles, andelectromagnetic vibrator means 110, similar to vibrator 42 on powdersupply hopper 39, flex and vibrate the sidewall 108 to preventaccumulation and compaction of the powder.

The duct 28 between powder separator housing 86 and recirculatedscreening bin 30 is of steeper slope than shown in FIG. 7 to assure thatthe overspray resin particles do not accumulate.

The dispensing means for new powder shown in FIG.

57U of the dynamometer type are mounted at different heights on thesidewall of powder supply hopper 39 and have paddle wheels 114 rotatablewithin supply hopper 39 by motor means 116. The output terminals ofupper bin level control S'IU are coupled to valve motor control 59, andthe output terminal of lower bin control 57L is connected by a conductor117 to the operating coil of relay 118. When the new and recirculatedoverspray powder within supply hopper 39 is below a first predeterminedamount, preferably 3 pounds, the paddle wheels 114L and 114U of both binlevel controls 57U and 57L turn freely, no signal is provided to valvemotor control 59, and control 59 energizes motor 52 through a resistance120 to rotate dump valve 47 at a relatively high speed to deliver newpowder to supply hopper 39. When the new and recirculated powder withinhopper 39 reaches a level corresponding to said first predeterminedamount, paddle wheel 114L of lower bin level control 57L is preventedfrom rotating by the powder, and control 57L provides an output to theoperating coil of relay 118 which actuates its movable contact 122relative to a pair of stationary contacts 123 connected to spaced tapson resistor 120 to thereby change the resistance in series with thefield winding of motor 52 and slow down motor 52 and the speed at whichdump valve 47 delivers new powder to supply hopper. When the new andrecirculated resin powder reaches a level corresponding to a secondpredetermined amount which may result in compaction of the powder,preferably five pounds, paddle wheel 114U of upper bin control 57U canno longer rotate, and upper bin level control 57U provides an outputsignal to valve motor control 59 which opens the energizing circuit tomotor 52 to thereby stop delivery of new powder to supply hopper 39before a sufficient amount of powder can accumulate in supply hopper 39to result in compaction of the powder. It will be appreciated that thedisclosed new powder dispensing means also constitutes means forminimizing the amount of resin powder in the dielectric spraycoating-system.

FIGS. 9 and schematically illustrate an embodiment of the inventionwhich permits a still further reduction in the amount of resin powder inthe electrostatic spray coating system and also further reduces thepossibility of shutdowns caused by obstructions in the system. Coarse oragglomerated resin particles and ferromagnetic particles are removedfrom the new powder before it is introduced into the coatingg system.Elements of the system of FIGS. 9 and 10 similar to those of theembodiments of FIGS. 1-6 and FIGS. 7-8 are given the same referencenumerals. The new powder hopper is eliminated in this embodiment, andnew dry resin powder is conveyed from a drum 124 by a flexible tubularconduit 125 enclosing a screw conveyor 128 rotatable within tubularconduit 125 by an electric motor 129. The feed inlet end of the screwconveyor conduit 125 is inserted into the new powder within drum 124,and the discharge end 131 thereof communicates with a duct 132registering with one inlet opening into a chamber 134. The chamber 134has a second inlet opening in an upper wall thereof communicating with aduct 28 from the powder separator housing 86 and also has an outletopening at its lower end communicating with recirculated powderscreening bin 30. The new resin powder delivered from drum 124 by screwconveyor conduit into duct 132 when motor 129 is energized falls bygravity onto the grate type hopper magnet 32 which attracts anyferromagnetic particles therein and then onto the inclined vibratingscreen 33, and any agglomerated resin particles or debris mixed with thenew powder do not pass through the screen 33 and slide down thevibrating screen 33 and fall into sight glass container 37.

The new powder particles and the recirculated powder particles whichpass through screen 33 fall into the cone-shaped powder supply hopper39. The powder level detector 136 of this embodiment preferably has anelongated vertical vibrating finger, or rod probe 137 positioned withinhopper 39 slightly above the bottom wall 140 thereof. The lower end ofrod probe 137 preferably depends below that level 142 of powder (seeFIG. 10) to which the resin particles fall due to gravity and thevibration of the walls of hopper 39 by the electromagnetic vibrators 42but above the powder level 144 in alignment with the apertures 145leading to the venturi chambers 60 in housings 61 into which aperturesthe powder is driven by negative pressure. Level detector 136 may bemounted on a bracket 147 affixed to a sidewall of recirculated powderscreening bin 30 and may be of the type sold under the trademarkDYNATROL described in Bulletin No. DJ-69 of Automation Products, Inc. ofHouston, Texas wherein an electric drive coil shown schematically at 138drives rod probe 137 into self-sustaining mechanical vibration at itsnatural resonant frequency when rod 137 is uncovered, and a pickup coilshown schematically at 139 is excited by the mechanical oscillations ofthe rod 137 and generates an alternating voltage signal when the levelof powder in bin 39 is low and rod 137 is uncovered. This low powderlevel signal may operate a relay (not shown) in motor control 146(illustrated in block form) which completes an energizing circuit tomotor 129 to rotate conveyor screw 128 and thus deliver new powder fromdrum 124 into chamber 134.

When the powder within hopper 39 approaches level 142, for example, oneinch to two inches above bottom wall 140, wherein the powder obstructsvibration of rod probe 137, dampening of the rod oscillations occurs,the magnitude of the rod oscillations is greatly reduced, and the outputfrom the pickup coil 139 drops to such a low level that motor control146 opens the energizing circuit to motor 129 to stop delivery of newpowder by screw conveyor 125 to chamber 134. This powder level detector136 is very sensitive and permits operation of the powder system of theinvention with only one or two pounds of resin powder in the entiresystem without the powder level falling to the point where portions ofthe articles 10 are not covered by the resin.

In this embodiment, the length of the legs 148 of the support table 149are shown to be of sufficient height so that the venturis 62 are raisedabove the level of the spray nozzles 17, thereby assuring that noU-shaped bends occur in the flexible hoses 70 between the venturis 62and the spray nozzles 17 in which the resin powder particles can becomecompacted and cause obstructions.

Two types of thermosetting resins are used in electrostatic spraysystems, namely, (l) the dry blend," or dry mix type wherein the variousresins and plasticizers are mixed separately in dry ball mills, and (2)the melt mix type wherein the resins, plasticizers and modifiers are allblended in liquid form, permitted to solidify upon cooling, and thenground into resin powder particles. The constituents of the dry blendtype tend to separate out in the electrostatic system, thereby alteringthe properties of the deposited resin coating, whereas each particle ofthe melt mix type resin is of uniform consistency and the constituentsare inseparable. The improved electrostatic spray system of theinvention is operable with resin powders of either the dry mix or of themelt mix type.

The disclosed electrostatic spray resin coating system preventsobstructions resulting from cornpactions of resin powder and therebysubstantially eliminates shutdowns of the apparatus. The disclosedapparatus does not require the continuous presence of an operator as wasnecessary with known resin coating equipment. The air surrounding thedisclosed electrostatic spray apparatus is not contaminated with theresin powder. Only a very minute amount of the resin powder is not usedin coating the workpiece in the disclosed apparatus. Further, thedisclosed electrostatic spray coating system supplies a uniform blend ofresin powder particle sizes to the spray nozzles at all times andproduces an optimum and uniform coating without voids and streaking andwhich has high dielectric strength.

While only a few embodiments of our invention have been illustrated anddescribed, many modifications and variations thereof will be readilyapparent to those skilled in the art, and consequently it should beunderstood that we do not intend to be limited to the particularembodiments shown and described.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. An electrostatic spray system for coating an article with resincomprising, in combination,

an electrostatic resin powder spray nozzle,

a supply hopper for new and recovered resin powder,

means communicating with said supply hopper for supplying resin powderto said nozzle,

means for electrostatically charging resin powder particles ejected fromsaid nozzle,

means for maintaining said article at a potential which will attractsaid charged resin particles to it,

means including a spray booth and powder separator means for recoveringoverspray resin powder particles ejected from said nozzles which do notcontact said article and for delivering them to said supply hopper,conveyor means for moving said article through said spray booth pastsaid nozzle,

means responsive to the presence of said article opposite said nozzlefor controlling said powder supplying means to deliver resin powder tosaid nozzle and responsive to the absence of said article opposite eachnozzle to shut off said powder supply, whereby a minimum of overspraypowder is circulated in said powder recovery system,

means for supplying new resin powder to said system,

and

means responsive to the level of new and recovered powder in said supplyhopper for controlling said new powder supplying means.

2. An electrostatic spray coating system in accordance with claim 1wherein said means for supplying new powder to said supply hopperincludes a hopper for new powder registering with said supply hopper, a

llti

powder dispensing valve between said new powder hopper and said supplyhopper, and wherein said means for controlling said new powder supplyingmeans includes powder level detecting means in said supply hopper foroperating said powder dispensing valve when the new and recovered powderin said supply hopper is below a predetermined level.

3. An electrostatic spray resin coating system in accordance with claim2 wherein said means for recovering overspray resin particles includes abin for recirculated powder disposed above said supply hopper, conduitmeans connecting said powder separator and said recirculated powder bin,and a vibrating screen in said recirculated powder bin disposed betweensaid conduit means and said supply hopper.

4. A system in accordance with claim 1 wherein said means for supplyingnew powder to said supply hopper includes a flexible conduit screwconveyor and a motor for driving said screw conveyor and said meansresponsive to the level of new and recovered powder controls theenergization of said motor.

5. A system in accordance with claim 4 wherein said means responsive tothe level of new and recovered powder includes a vibratory rod probedepending to a predetermined level in said supply hopper, electric exciter coil means for inducing sustained oscillations in said rod probewhen it is uncovered, and electric pickup coil means for generating anoutput signal in response to oscillations of said rod probe.

6. An electrostatic spray coating system in accordance with claim 1wherein said powder separator includes filter means on which saidoverspray resin particles can temporarily collect and means forintermittently cleaning said filter means and said means for recoveringoverspray particles includes screening means disposed between saidpowder separator and said supply hopper for removingcoarse,'agglomerated particles from said powder and the discharge end ofsaid means for supplying new powder is above said screening means.

7. A system in accordance with claim 6, wherein said means forrecovering overspray resin particles includes means for creatingnegative pressure in said powder separator to draw overspray resinparticles into said separator and continuously operating valve meansdisposed between said powder separator and said screening means.

8. A system in accordance with claim 7 wherein said supply hopper has asteeply sloping flexible sidewall and vibratory means for flexing saidsidewalk 9. An electrostatic spray coating system in accordance withclaim 6 wherein said powder separator includes a housing, a plurality ofdepending inflatable bags within said housing, and means forintermittently inflating said bags.

10. An electrostatic spray system for coating an article with resincomprising, in combination,

an electrostatic resin powder spray nozzle,

a supply hopper for new and recovered resin powder,

means communicating with said supply hopper for supplying resin powderto said nozzle,

means for electrostatically charging resin powder particles ejected fromsaid nozzle,

means for maintaining said article at a potential which will attractsaid charged resin particles to it,

means including a spray booth for recovering overspray resin powderparticles ejected from said spray nozzle which do not contact saidarticle and for delivering them to said supply hopper,

means for feeding new powder to said supply hopper,

a motor for actuating said new powder feeding means, level detectingmeans responsive to the level of new and overspray resin powder in saidsupply hopper for controlling the energization of said motor,

means responsive to the presence of said article opposite said nozzlefor controlling said powder supplying means to deliever resin powder tosaid noz zle and responsive to the absence of said article opposite saidnozzle to shut off said powder supply whereby a minimum of overspraypowder is circulated in said powder recovering means, and

conveyor means for moving said article through said spray booth pastsaid nozzle.

11. A system in accordance with claim wherein said means for recoveringoverspray resin particles includes a vibratory screen above said supplyhopper for removing coarse agglomerated particles from said powder.

12. A system in accordance with claim 11 wherein said means for feedingnew powder includes a flexible tubular conduit having its discharge endabove said vibratory screen, a screw conveyor in said flexible conduit,and a motor for driving said screw conveyor, and wherein said powderlevel detecting means controls the energization of said motor.

13. A system in accordance with claim 11 wherein said level detectingmeans includes a vibratory rod probe depending to a predetermined levelin said supply hopper, electric exciter coil means for generatingsustained oscillations in said rod probe when it is uncovered, andelectric pickup coil means for generating an output signal in responseto the oscillations in said rod probe.

14. An electrostatic spray coating system in accordance with claim 10wherein said means for supplying resin powder to said nozzle includesventuri means communicating with said supply hopper, conduit meansbetween said venturi means and said nozzle, a source of pressurized air,and valve means between said pressurized air source and said venturimeans controlled by said means responsive to the presence of saidarticle opposite said nozzle.

15. An electrostatic spray system for coating an article with resincomprising, in combination,

an electrostatic resin powder spray nozzle,

a supply hopper for new and recovered resin powder,

means communicating with said supply hopper for supplying resin powderto said nozzle,

means for electrostatically charging resin powder particles ejected fromsaid nozzle,

means for maintaining said article at a potential which will attractsaid charged resin particles to it,

means for recovering overspray resin powder particles ejected from saidspray nozzle which do not contact said article and for delivering themto said supply hopper,

means responsive to the absence of said article opposite said nozzle forcontrolling said powder supplying means to deliver resin powder to saidnozzle and responsive to the absence of said article opposite saidnozzle to shut off said powder supply, whereby a minimum of overspraypowder is'circulated in said powder recovery means,

means for feeding new powder to said supply hopper,

and

level detecting means responsive to the level of new and recoveredpowder in said supply hopper for controlling said means for feeding newpowder;

16. An electrostatic spray resin coating system in accordance with claim15 wherein said means for recovering overspray resin powder particlesincludes screening means above said supply hopper for removing coarseparticles from said powder and wherein said means for feeding new powderhas its discharge end above said screening means.

17. An electrostatic spray coating system in accordance with claim 16wherein said means for feeding new powder includes a flexible conduithaving its discharge end above said screening means, a screw conveyorwithin said flexible conduit, a motor for driving said screw conveyor,and wherein said powder level detecting means controls the energizationof said motor.

18. An electrostatic spray coating system in accordance with claim 16wherein said powder level detecting means includes a vibratory rod probedepending to a predetermined level within said supply hopper, electricexciter coil means for generating sustained oscillations in said rodprobe when it is uncovered, and electric pickup coil means forgenerating an output signal in response to the oscillations in said rodprobe.

19. A system according to claim 16 wherein said means for supplyingpowder to said nozzle includes venturi means communicating with saidsupply hopper, a conduit between said venturi means and said nozzle, asource of pressurized air for said venturi means, air valve means forsupplying and shutting off said source of pressurized air to saidventuri means, and wherein said means responsive to the presence of saidarticle controls said air valve means.

20. A system according to claim 16 wherein said means-for recoveringoverspray resin particles includes a spray booth and said meansresponsive to the presence of said article opposite said nozzle includesconveyor means for moving said article through said booth past saidnozzle.

21. A system according to claim 20 wherein said means for recoveringoverspray resin powder particles includes a powder separator having ahousing, a plurality ofdepending inflatable bags within said housing,and means for intermittently inflating said bags.

22. A system according to claim 21 and including exhaust fan means onsaid powder separator housing for creating negative pressure to drawoverspray resin powder particles into said separator housing andcontinuously operating valve means at the lower end of said separatorhousing.

23. A system according to claim 22 wherein said powder separator housinghas a flexible steeply sloping sidewall at its lower end above saidcontinuously operating valve means and vibratory means for flexing saidseparator housing sidewall.

24. An electrostatic spray system for coating an article with resincomprising, in combination,

a spray booth,

a resin powder spray nozzle within said booth,

conveyor means for moving an article to be coated through said boothpast said nozzle,

means for electrically charging resin powder particles ejected from saidnozzle,

means for maintaining said article in an electric potential which willattract said charged resin particles to it,

a powder separator,

means including first conduit means between said spray booth and saidpowder separator for recovering overspray resin particles ejected fromsaid nozzle which do not contact said article and for delivering them tosaid powder separator,

a supply hopper for new and recovered powder,

second conduit means communicating at one end with said powder separatorand at the other end with said supply hopper,

means including a venturi communicating with said supply hopper and asource of pressurized air for said venturi for supplying powder fromsaid supply hopper to said nozzle,

air valve means for selectively connecting and shutting off said sourceof pressurized air to said venturi,

means responsive to the presence of said article opposite said nozzlefor opening said air valve means and to the absence of said articleopposite said nozzle for closing said air valve means,

said powder separator having a continuously operating outlet valveleading to said second conduct means, exhaust fan means for creatingnegative pressure within said separator to draw overspray resinparticles into said separator, a steeply sloping flexible sidewall atits lower end and above said outlet valve, vibratory means for flexingsaid sidewall to prevent compaction of said overspray resin powderparticles, a plurality of depending inflatable bags in which saidoverspray resin powder particles settle, and means for intermittentlyinflating said bags.

25. An electrostatic spray system for coating an article with resincomprising, in combination,

a spray booth,

a resin powder spray nozzle within said booth,

conveyor means for moving an article to be coated through said boothpast said nozzle, r

means for electrically charging resin powder particles ejected from saidnozzle,

means for maintaining said article at an electric potential which willattract said charged resin particles to it,

a powder separator,

means including first conduit means between said spray booth and saidpowder separator for recovering overspray resin particles ejected fromsaid nozzle which do not contact said article and for delivering them tosaid powder separator,

a supply hopper for new and recovered powder,

second conduit means communicating at one end with said powder separatorand at the other end with said supply hopper,

means including a venturi communicating with said supply hopper and asource of pressurized air for said venturi for supplying powder fromsaid supply hopper to said nozzle,

means responsive to the'presence of said article opposite said nozzlefor opening said air valve means and to the absence of said articleopposite said nozzle for closing said air valve means,

screening means between said powder separator and said supply hopper forremoving coarse agglomerated particles from said powder,

means for supplying new powder to said supply hopper and powder leveldetecting means responsive to the amount of new and overspray powder insaid supply hopper for controlling said new powder supplying means.

26. A system in accordance with claim 25 wherein said new powdersupplying means includes a bin for new powder, powder dispensing valvemeans between said new powder bin and said supply hopper, and saidpowder level detecting means controls said powder dispensing valvemeans.

27. A system in accordance with claim 26 wherein said powder dispensingvalve means rotates to dispsense new powder to said supply hopper andincluding variable speed motor means for driving said powder dispensingvalve means and means responsive to the amount of new and recirculatedpowder in said supply hopper for selectively varying the speed of saidmotor means.

28. A system in accordance with claim 25 wherein said means forsupplying new powder includes a flexible conduit screw conveyor havingits discharge end above said screening means, and a motor for drivingsaid screw conveyor and said powder level detecting means controls theenergization of said motor and includes a vibratory rod probe dependingto a predetermined level within said supply hopper, electric excitercoil means for inducing sustained oscillations in said rod probe whensaid probe is uncovered, and electric pickup coil means for generatingan output signal in response to the oscillations in said rod probe.

29. A system in accordance with claim 25 wherein said supply hopper hasa steeply sloping flexible sidewall and including vibratory means forflexing said sidewall to prevent compaction of said powder in saidsupply hopper.

30. An electrostatic spray system for coating an article with resincomprising, in combination,

an electrosattic resin powder spray nozzle,

a supply hopper for new and recovered resin powder,

means communicating with said supply hopper for supplying resin powderto said nozzle,

means for electrostatically charging resin powder particles ejected fromsaid nozzle,

means for maintaining said article at a potential which will attractsaid charged resin particles to it,

means for recovering overspray resin powder particles ejected from saidspray nozzle which do not contact said article and for delivering themto said supply hopper and including a spray booth and powder separatormeans for returning overspray powder particles to said supply hopper,said powder separator means having filter means on which said oversprayresin powder particles can temporarily collect and means forintermittently cleaning said filter means, and

means responsive to the presence of said article opposite said nozzlefor controlling said powder supplying means to deliver resin powder tosaid nozzle and responsive to the absence of said article opposite saidnozzle to shut off said powder supply, whereby a minimum of overspraypowder is circulated in said powder recovering means.

31. An electrostatic spray system for coating an article with resincomprising, in combination,

ing overspray resin particles ejected from said nozzle which do notcontact said article and for delivering them to said powder separator,filter means on which said overspray resin powder particles cantemporarily collect, and means for intermittently cleaning said filtermeans,

a supply hopper for new and recovered powder,

second conduit means communicating at one end with said powder separatorand at the other end with said supply hopper,

means including a venturi communicating with said supply hopper and asource of pressurized air for said venturi for supplying powder fromsaid supply hopper to said nozzle,

air valve means for selectively connecting and shutting off said sourceof pressurized air to said venturi, and

means responsive to the presence of said article opposite said nozzlefor opening said air valve means and to the absence of said articleopposite said nozzle for closing said air valve means.

1. An electrostatic spray system for coating an article with resincomprising, in combination, an electrostatic resin powder spray nozzle,a supply hopper for new and recovered resin powder, means communicatingwith said supply hopper for supplying resin powder to said nozzle, meansfor electrostatically charging resin powder particles ejected from saidnozzle, means for maintaining said article at a potential which willattract said charged resin particles to it, means including a spraybooth and powder separator means for recovering overspray resin powderparticles ejected from said nozzles which do not contact said articleand for delivering them to said supply hopper, conveyor means for movingsaid article through said spray booth past said nozzle, means responsiveto the presence of said article opposite saId nozzle for controllingsaid powder supplying means to deliver resin powder to said nozzle andresponsive to the absence of said article opposite each nozzle to shutoff said powder supply, whereby a minimum of overspray powder iscirculated in said powder recovery system, means for supplying new resinpowder to said system, and means responsive to the level of new andrecovered powder in said supply hopper for controlling said new powdersupplying means.
 2. An electrostatic spray coating system in accordancewith claim 1 wherein said means for supplying new powder to said supplyhopper includes a hopper for new powder registering with said supplyhopper, a powder dispensing valve between said new powder hopper andsaid supply hopper, and wherein said means for controlling said newpowder supplying means includes powder level detecting means in saidsupply hopper for operating said powder dispensing valve when the newand recovered powder in said supply hopper is below a predeterminedlevel.
 3. An electrostatic spray resin coating system in accordance withclaim 2 wherein said means for recovering overspray resin particlesincludes a bin for recirculated powder disposed above said supplyhopper, conduit means connecting said powder separator and saidrecirculated powder bin, and a vibrating screen in said recirculatedpowder bin disposed between said conduit means and said supply hopper.4. A system in accordance with claim 1 wherein said means for supplyingnew powder to said supply hopper includes a flexible conduit screwconveyor and a motor for driving said screw conveyor and said meansresponsive to the level of new and recovered powder controls theenergization of said motor.
 5. A system in accordance with claim 4wherein said means responsive to the level of new and recovered powderincludes a vibratory rod probe depending to a predetermined level insaid supply hopper, electric exciter coil means for inducing sustainedoscillations in said rod probe when it is uncovered, and electric pickupcoil means for generating an output signal in response to oscillationsof said rod probe.
 6. An electrostatic spray coating system inaccordance with claim 1 wherein said powder separator includes filtermeans on which said overspray resin particles can temporarily collectand means for intermittently cleaning said filter means and said meansfor recovering overspray particles includes screening means disposedbetween said powder separator and said supply hopper for removingcoarse, agglomerated particles from said powder and the discharge end ofsaid means for supplying new powder is above said screening means.
 7. Asystem in accordance with claim 6, wherein said means for recoveringoverspray resin particles includes means for creating negative pressurein said powder separator to draw overspray resin particles into saidseparator and continuously operating valve means disposed between saidpowder separator and said screening means.
 8. A system in accordancewith claim 7 wherein said supply hopper has a steeply sloping flexiblesidewall and vibratory means for flexing said sidewall.
 9. Anelectrostatic spray coating system in accordance with claim 6 whereinsaid powder separator includes a housing, a plurality of dependinginflatable bags within said housing, and means for intermittentlyinflating said bags.
 10. An electrostatic spray system for coating anarticle with resin comprising, in combination, an electrostatic resinpowder spray nozzle, a supply hopper for new and recovered resin powder,means communicating with said supply hopper for supplying resin powderto said nozzle, means for electrostatically charging resin powderparticles ejected from said nozzle, means for maintaining said articleat a potential which will attract said charged resin particles to it,means including a spray booth for recovering overspray resin powderparticles ejected from said spray nozzle which do not contact saidarticle and for Delivering them to said supply hopper, means for feedingnew powder to said supply hopper, a motor for actuating said new powderfeeding means, level detecting means responsive to the level of new andoverspray resin powder in said supply hopper for controlling theenergization of said motor, means responsive to the presence of saidarticle opposite said nozzle for controlling said powder supplying meansto deliever resin powder to said nozzle and responsive to the absence ofsaid article opposite said nozzle to shut off said powder supply wherebya minimum of overspray powder is circulated in said powder recoveringmeans, and conveyor means for moving said article through said spraybooth past said nozzle.
 11. A system in accordance with claim 10 whereinsaid means for recovering overspray resin particles includes a vibratoryscreen above said supply hopper for removing coarse agglomeratedparticles from said powder.
 12. A system in accordance with claim 11wherein said means for feeding new powder includes a flexible tubularconduit having its discharge end above said vibratory screen, a screwconveyor in said flexible conduit, and a motor for driving said screwconveyor, and wherein said powder level detecting means controls theenergization of said motor.
 13. A system in accordance with claim 11wherein said level detecting means includes a vibratory rod probedepending to a predetermined level in said supply hopper, electricexciter coil means for generating sustained oscillations in said rodprobe when it is uncovered, and electric pickup coil means forgenerating an output signal in response to the oscillations in said rodprobe.
 14. An electrostatic spray coating system in accordance withclaim 10 wherein said means for supplying resin powder to said nozzleincludes venturi means communicating with said supply hopper, conduitmeans between said venturi means and said nozzle, a source ofpressurized air, and valve means between said pressurized air source andsaid venturi means controlled by said means responsive to the presenceof said article opposite said nozzle.
 15. An electrostatic spray systemfor coating an article with resin comprising, in combination, anelectrostatic resin powder spray nozzle, a supply hopper for new andrecovered resin powder, means communicating with said supply hopper forsupplying resin powder to said nozzle, means for electrostaticallycharging resin powder particles ejected from said nozzle, means formaintaining said article at a potential which will attract said chargedresin particles to it, means for recovering overspray resin powderparticles ejected from said spray nozzle which do not contact saidarticle and for delivering them to said supply hopper, means responsiveto the absence of said article opposite said nozzle for controlling saidpowder supplying means to deliver resin powder to said nozzle andresponsive to the absence of said article opposite said nozzle to shutoff said powder supply, whereby a minimum of overspray powder iscirculated in said powder recovery means, means for feeding new powderto said supply hopper, and level detecting means responsive to the levelof new and recovered powder in said supply hopper for controlling saidmeans for feeding new powder;
 16. An electrostatic spray resin coatingsystem in accordance with claim 15 wherein said means for recoveringoverspray resin powder particles includes screening means above saidsupply hopper for removing coarse particles from said powder and whereinsaid means for feeding new powder has its discharge end above saidscreening means.
 17. An electrostatic spray coating system in accordancewith claim 16 wherein said means for feeding new powder includes aflexible conduit having its discharge end above said screening means, ascrew conveyor within said flexible conduit, a motor for driving saidscrew conveyor, and wherein said powder level detecting means controlsthe energization of said motor.
 18. An electrostatic spray coatingsystem in accordance with claim 16 wherein said powder level detectingmeans includes a vibratory rod probe depending to a predetermined levelwithin said supply hopper, electric exciter coil means for generatingsustained oscillations in said rod probe when it is uncovered, andelectric pickup coil means for generating an output signal in responseto the oscillations in said rod probe.
 19. A system according to claim16 wherein said means for supplying powder to said nozzle includesventuri means communicating with said supply hopper, a conduit betweensaid venturi means and said nozzle, a source of pressurized air for saidventuri means, air valve means for supplying and shutting off saidsource of pressurized air to said venturi means, and wherein said meansresponsive to the presence of said article controls said air valvemeans.
 20. A system according to claim 16 wherein said means forrecovering overspray resin particles includes a spray booth and saidmeans responsive to the presence of said article opposite said nozzleincludes conveyor means for moving said article through said booth pastsaid nozzle.
 21. A system according to claim 20 wherein said means forrecovering overspray resin powder particles includes a powder separatorhaving a housing, a plurality of depending inflatable bags within saidhousing, and means for intermittently inflating said bags.
 22. A systemaccording to claim 21 and including exhaust fan means on said powderseparator housing for creating negative pressure to draw overspray resinpowder particles into said separator housing and continuously operatingvalve means at the lower end of said separator housing.
 23. A systemaccording to claim 22 wherein said powder separator housing has aflexible steeply sloping sidewall at its lower end above saidcontinuously operating valve means and vibratory means for flexing saidseparator housing sidewall.
 24. An electrostatic spray system forcoating an article with resin comprising, in combination, a spray booth,a resin powder spray nozzle within said booth, conveyor means for movingan article to be coated through said booth past said nozzle, means forelectrically charging resin powder particles ejected from said nozzle,means for maintaining said article in an electric potential which willattract said charged resin particles to it, a powder separator, meansincluding first conduit means between said spray booth and said powderseparator for recovering overspray resin particles ejected from saidnozzle which do not contact said article and for delivering them to saidpowder separator, a supply hopper for new and recovered powder, secondconduit means communicating at one end with said powder separator and atthe other end with said supply hopper, means including a venturicommunicating with said supply hopper and a source of pressurized airfor said venturi for supplying powder from said supply hopper to saidnozzle, air valve means for selectively connecting and shutting off saidsource of pressurized air to said venturi, means responsive to thepresence of said article opposite said nozzle for opening said air valvemeans and to the absence of said article opposite said nozzle forclosing said air valve means, said powder separator having acontinuously operating outlet valve leading to said second conductmeans, exhaust fan means for creating negative pressure within saidseparator to draw overspray resin particles into said separator, asteeply sloping flexible sidewall at its lower end and above said outletvalve, vibratory means for flexing said sidewall to prevent compactionof said overspray resin powder particles, a plurality of dependinginflatable bags in which said overspray resin powder particles settle,and means for intermittently inflating said bags.
 25. An electrostaticspray system for coating an article with resin comprising, incombiNation, a spray booth, a resin powder spray nozzle within saidbooth, conveyor means for moving an article to be coated through saidbooth past said nozzle, means for electrically charging resin powderparticles ejected from said nozzle, means for maintaining said articleat an electric potential which will attract said charged resin particlesto it, a powder separator, means including first conduit means betweensaid spray booth and said powder separator for recovering oversprayresin particles ejected from said nozzle which do not contact saidarticle and for delivering them to said powder separator, a supplyhopper for new and recovered powder, second conduit means communicatingat one end with said powder separator and at the other end with saidsupply hopper, means including a venturi communicating with said supplyhopper and a source of pressurized air for said venturi for supplyingpowder from said supply hopper to said nozzle, air valve means forselectively connecting and shutting off said source of pressurized airto said venturi, means responsive to the presence of said articleopposite said nozzle for opening said air valve means and to the absenceof said article opposite said nozzle for closing said air valve means,screening means between said powder separator and said supply hopper forremoving coarse agglomerated particles from said powder, means forsupplying new powder to said supply hopper and powder level detectingmeans responsive to the amount of new and overspray powder in saidsupply hopper for controlling said new powder supplying means.
 26. Asystem in accordance with claim 25 wherein said new powder supplyingmeans includes a bin for new powder, powder dispensing valve meansbetween said new powder bin and said supply hopper, and said powderlevel detecting means controls said powder dispensing valve means.
 27. Asystem in accordance with claim 26 wherein said powder dispensing valvemeans rotates to dispsense new powder to said supply hopper andincluding variable speed motor means for driving said powder dispensingvalve means and means responsive to the amount of new and recirculatedpowder in said supply hopper for selectively varying the speed of saidmotor means.
 28. A system in accordance with claim 25 wherein said meansfor supplying new powder includes a flexible conduit screw conveyorhaving its discharge end above said screening means, and a motor fordriving said screw conveyor and said powder level detecting meanscontrols the energization of said motor and includes a vibratory rodprobe depending to a predetermined level within said supply hopper,electric exciter coil means for inducing sustained oscillations in saidrod probe when said probe is uncovered, and electric pickup coil meansfor generating an output signal in response to the oscillations in saidrod probe.
 29. A system in accordance with claim 25 wherein said supplyhopper has a steeply sloping flexible sidewall and including vibratorymeans for flexing said sidewall to prevent compaction of said powder insaid supply hopper.
 30. An electrostatic spray system for coating anarticle with resin comprising, in combination, an electrosattic resinpowder spray nozzle, a supply hopper for new and recovered resin powder,means communicating with said supply hopper for supplying resin powderto said nozzle, means for electrostatically charging resin powderparticles ejected from said nozzle, means for maintaining said articleat a potential which will attract said charged resin particles to it,means for recovering overspray resin powder particles ejected from saidspray nozzle which do not contact said article and for delivering themto said supply hopper and including a spray booth and powder separatormeans for returning overspray powder particles to said supply hopper,said powder separator means having filter means on which said oversprayresin powder partIcles can temporarily collect and means forintermittently cleaning said filter means, and means responsive to thepresence of said article opposite said nozzle for controlling saidpowder supplying means to deliver resin powder to said nozzle andresponsive to the absence of said article opposite said nozzle to shutoff said powder supply, whereby a minimum of overspray powder iscirculated in said powder recovering means.
 31. An electrostatic spraysystem for coating an article with resin comprising, in combination, aspray booth, a resin powder spray nozzle within said booth, conveyormeans for moving an article to be coated through said booth past saidnozzle, means for electrically charging resin powder particles ejectedfrom said nozzle, means for maintaining said article at an electricpotential which will attract said charged resin particles to it, apowder separator, means including first conduit means between said spraybooth and said powder separator for recovering overspray resin particlesejected from said nozzle which do not contact said article and fordelivering them to said powder separator, filter means on which saidoverspray resin powder particles can temporarily collect, and means forintermittently cleaning said filter means, a supply hopper for new andrecovered powder, second conduit means communicating at one end withsaid powder separator and at the other end with said supply hopper,means including a venturi communicating with said supply hopper and asource of pressurized air for said venturi for supplying powder fromsaid supply hopper to said nozzle, air valve means for selectivelyconnecting and shutting off said source of pressurized air to saidventuri, and means responsive to the presence of said article oppositesaid nozzle for opening said air valve means and to the absence of saidarticle opposite said nozzle for closing said air valve means.